Warmer Himalayas 'suffocates fish in Arabian Sea'

Reduced snowfall in the Himalayas, caused by global warming, is threatening marine life in the distant Arabian Sea, say researchers.

The team, led by Joaquim Goes of the Bigelow Laboratory for Ocean Sciences, United States, says the phenomenon could also aggravate global climate change.

During the northern hemisphere's summer months, the difference in temperature between the land and sea surfaces in Eurasia creates strong monsoon winds that blow from the Arabian Sea eastwards towards India.

These winds pull surface water with them. In the western Arabian Sea, near the coasts of Oman, Somalia and Yemen, the displaced surface water is replaced by water rising from the bottom of the Arabian Sea.

The deeper, cooler water is rich in nutrients. Their upward movement leads to a surge in the number of microscopic plants, called phytoplankton, that float near the surface.

In a paper published in Science today (22 April), Goes's team shows that the amount of phytoplankton at the surface of the Arabian Sea has been gradually increasing every year since the late 1990s.

The researchers found that this was linked to changes in the strength of the winds and temperature of the sea surface in the western Arabian Sea, which they monitored over the same period.

They conclude that the recent increase in phytoplankton is caused by stronger monsoon winds dragging up more of the nutrient-rich water from the bottom of the Arabian Sea.

They believe this is because the amount of winter snowfall in the Himalayas has been decreasing since the late 1990s. As a result, the land heats up faster during the summer months generating stronger winds.

More phytoplankton might seem like a good thing. The tiny plants are at the bottom of the food chain, so more of them should support more marine animals, meaning there is more food in the food chain.

The trouble, explains Goes, is that while the deep water is rich in nutrients, it has little oxygen. Dragging more of it up to the surface is suffocating marine life. Indeed, fishermen off the coasts of Somalia, Oman and Yemen say the number of dead fish they catch has been increasing steadily for the past five to seven years.

If the trend continues, more fish will die, says Goes.

But the problems do not stop there. Bringing deep water to the surface in this part of the globe could also dramatically increase global warming.

Some bacteria have found a way of dealing with the low oxygen levels at the bottom of the Arabian Sea. They do this by extracting oxygen from nitrate, which is found in the water. The process, known as 'de-nitrification', produces nitrous oxide, a powerful greenhouse gas.

When de-nitrification happens at the bottom of the sea, the nitrous oxide is trapped. But when the water rises to the surface, the gas is released into the atmosphere. There, it acts as a greenhouse gas that is about 300 times more harmful to the environment than carbon dioxide.

If the Himalayas continue to receive less winter snow, says Goes, "the Arabian Sea will become a chimney for nitrous oxide". And that could mean that climate change would be much worse than is currently anticipated.

Map of the Arabian Sea showing the direction of the summer monsoon winds